An interesting question was raised in the WSPR forum: If my WSPR beacon is received by another station, and I receive his beacon, can we claim a contact and exchange QSLs?
I had been thinking about this myself a couple of days before the question was raised. WSPR is a good mode for HF propagation studies but it has become a popular activity with people who like to see what can be achieved with very little power. It is also popular with those who have no opportunity to put up good antennas who find that despite this they can still experience the thrill of sending radio signals half way round the world. So it's easy to see why people would like to claim contacts using WSPR.
The answer given, with which I must reluctantly agree, is that no, reception of each other's beacons does not constitute a contact because the confirmation of contact is only sent via the internet and not over the radio. WSPRers can QSL reception reports if they want, but these cards should be treated as SWL reports not two-way QSOs, and will not be valid for awards.
There is a WSPR QSO mode, but I have hardly ever heard anyone using it. The reason, I suspect, is that it is fatally flawed and unusable. WSPR works in synchronized two-minute cycles, of which about one minute and 54 seconds is spent transmitting information. The information is not decoded in real-time, it is decoded after the transmission period is over. This means that in theory you have five seconds to react to seeing someone's CQ call and set up your reply to them.
In practise you have less than that, because it takes at least a couple of seconds before you see whose signals have been decoded. Sometimes you have literally less than no time, because if there are a lot of WSPR signals it can take the computer more than five seconds to decode everything that was heard. Because the start of transmission time is critical you would have to wait best part of two minutes before you could send your reply. By that time the other station, seeing no response, might send his CQ again so he won't receive your reply anyway. It's a test of patience that is likely to end in frustration and I'm not surprised that the QSO mode receives little use.
It's a shame, but I don't see a solution without speeding up the transmission period so it takes 1m45s, or rewriting the software so that it does at least a quick decode of stronger signals in real time. There are other JT modes that can be used for QSOs, though they don't work with such weak signals and are unlikely to get you round the world with 2 watts to a curtain rail on a day with no sunspots.